Function push_slack_message_into_chat 

pub(crate) fn push_slack_message_into_chat(
    workspace: &Workspace,
    target_key: &SessionSlot,
    prose: &str,
    uuid: &str,
)

Emit a typed SlackMessageAppended so the target session’s chat buffer shows the inbound Slack block. Mirrors push_gotify_notification_into_chat - the target’s claude subprocess still receives the prose via a separate Command::Prompt dispatch; this only drives the visible chat echo.